Synoptic Analysis of Summer Temperature Extremes and the Role of the Dasht-e Kavir in Semnan Province, Iran

Extended Abstract

Introduction

Extreme temperature events, particularly summer heat waves, are among the most significant climate-related hazards affecting arid and semi-arid regions worldwide. These events have intensified in recent decades due to global warming and regional climatic variability, leading to severe impacts on human health, water resources, ecosystems, agriculture, and energy demand. Iran, located within the subtropical dry belt, is highly vulnerable to temperature extremes, especially during the warm season. Among Iranian provinces, Semnan Province holds a distinctive geographical position, extending from the southern slopes of the Alborz Mountains to the northern margins of the Dasht-e Kavir desert. This transitional setting creates favorable conditions for the development and persistence of extreme heat events.The proximity of southern Semnan to the Dasht-e Kavir plays a critical role in regional thermal dynamics. The desert surface, characterized by sparse vegetation, low soil moisture, low albedo, and high heat capacity, intensifies surface heating and strengthens thermal low-pressure systems. These surface processes interact with large-scale atmospheric circulation patterns, enhancing atmospheric stability and suppressing vertical mixing, which in turn contributes to the persistence of heat waves.Although numerous studies have examined heat waves and synoptic patterns over Iran, limited attention has been paid to the combined role of synoptic-scale circulation and underlying surface characteristics, particularly the influence of major desert areas such as the Dasht-e Kavir. Therefore, the primary objective of this study is to identify and analyze the dominant synoptic patterns responsible for summer temperature extremes in Semnan Province and to clarify the role of the Dasht-e Kavir in intensifying and expanding these extreme thermal conditions.

Methodology

This study is based on daily maximum temperature data recorded at 26 synoptic and climatological stations located in Semnan Province and its surrounding areas over a 34-year period (1987–2020). To identify summer heat waves, a visual (expert-based) detection method was applied, considering the persistence of abnormally high temperatures over several consecutive days and significant positive deviations from long-term climatological means. Only events occurring during the summer season (June–August) were selected for analysis.For each identified heat wave, the peak day—defined as the day with the highest spatially averaged maximum temperature—was selected for synoptic analysis. Atmospheric circulation data were obtained from the NCEP/NCAR reanalysis dataset, including sea-level pressure, geopotential height, air temperature, and wind fields. Synoptic maps were generated for sea level and pressure levels of 1000, 925, 850, 700, and 500 hPa using the GrADS software environment. Hosseinan station was selected as the representative (index) station due to its frequent exposure to extreme temperatures, high thermal intensity, and close proximity to the Dasht-e Kavir. Composite analysis was conducted to classify heat wave events based on similarities in synoptic configurations. The resulting circulation patterns were then interpreted in relation to both large-scale atmospheric dynamics and local surface conditions, particularly desert-related thermal effects.

Results and Discussion

The results indicate that a total of 26 summer heat waves occurred in Semnan Province during the study period, with the highest frequency observed in July and August. These heat waves varied in duration and intensity but consistently exhibited strong spatial coherence across the southern and central parts of the province.Synoptic classification revealed two dominant circulation patterns associated with these extreme temperature events. The most frequent and influential pattern, accounting for approximately 69% of all heat waves, is characterized by a combined thermal low-pressure system over Pakistan and the Arabian Peninsula. In this pattern, intense thermal lows dominate the lower troposphere (1000, 925, and 850 hPa), while a pronounced subtropical ridge and anticyclonic circulation are established in the mid-troposphere (700 and 500 hPa). This vertical configuration promotes strong atmospheric subsidence, adiabatic warming, and reduced cloud cover, leading to enhanced solar radiation and surface heating. The persistence of the mid-level anticyclone suppresses vertical air exchange, allowing heat to accumulate near the surface over several consecutive days. The Dasht-e Kavir significantly reinforces this mechanism by intensifying surface sensible heat fluxes and strengthening the thermal low over central Iran, thereby extending extreme heat conditions toward Semnan Province. The second identified pattern involves a combined thermal low extending from the Arabian Peninsula toward the Lut Desert and southern Turkmenistan. While this pattern can generate very high temperatures during certain events, it is generally less persistent due to weaker mid-tropospheric support and occasional intrusion of dynamic disturbances at higher levels. As a result, its contribution to long-lasting heat waves in Semnan Province is comparatively limited. Overall, the findings highlight the critical interaction between synoptic-scale circulation systems and land surface characteristics. The Dasht-e Kavir acts not merely as a passive background feature but as an active thermal driver that amplifies and sustains extreme temperature conditions through land–atmosphere feedback mechanisms.

Conclusion

This study demonstrates that summer temperature extremes in Semnan Province are primarily controlled by specific synoptic circulation patterns, particularly the combined thermal low-pressure system over Pakistan and the Arabian Peninsula coupled with a mid-tropospheric subtropical anticyclone. The Dasht-e Kavir plays a fundamental role in intensifying these heat waves by enhancing surface heating, reinforcing low-level thermal lows, and promoting atmospheric stability.The results emphasize the necessity of incorporating surface characteristics, especially major desert regions, into synoptic and climatological analyses of extreme heat events. Improved understanding of these mechanisms can contribute to the development of early warning systems, risk management strategies, and climate adaptation planning in arid and semi-arid regions of Iran.

Keywords: Heat Waves; Temperature Extremes; Synoptic Patterns; Dasht-e Kavir; Semnan Province; Iran